The present invention relates to a power brake device for wheeled vehicles, and more particularly to a pneumatically operated brake booster of the type which comprises a hydraulic cylinder disposed within a hydraulic braking circuit connecting a master cylinder to wheel brake cylinders for increasing the braking pressure trapped within the wheel brake cylinders and a servomotor for pneumatically activating the hydraulic cylinder in response to hydraulic pressure from the master cylinder.
In a conventional brake booster of this kind, the interior of the servomotor is subdivided by a spring loaded diaphragm piston into a negative pressure chamber and a variable pressure chamber, the former being in connection to a source of negative pressure and the latter connected to the atmospheric pressure in response to master cylinder pressure applied to the hydraulic cylinder from the master cylinder. The diaphragm piston is operatively connected to an operating piston reciprocable within the cylinder to increase the braking pressure trapped within the wheel brake cylinders. In operation of the brake booster, the diaphragm piston of the servomotor is operated by a difference between the negative and atmospheric pressures to activate the hydraulic cylinder. Thus, the increasing ratio of the braking pressure is inevitably influenced by the operation of the diaphragm piston. This means that under a heavy loaded condition of the vehicle, the braking pressure becomes insufficient and that any failure of the vacuum pump in supplying negative pressure to the negative pressure chamber will cause disastrous consequences due to reduction of the braking pressure at the wheel brake cylinders.